The present invention relates to the improvement of metallic-ceramic substrates for high-voltage modules.
Power semiconductor components, particularly IGBTs, are fixed on a metallic-ceramic substrate. FIG. 2 herein shows a typical unit construction in cross section. The utilized substrate comprises a ceramic layer 1 that normally is comprised of Al.sub.2 O.sub.3 or AlN, for example. Metal layers that are formed by a copper metallization, for example, are situated on both main sides 21, 22 of this ceramic layer. Given the installation, a bottom metal layer 3 is attached to a bottom plate 5 by means of soldered joint 4, wherein the bottom plate 5 can be made of copper. The power semiconductor components 15, such as an IGBT or a diode, are attached to an upper metal layer 2 by means of a further soldered joint 12. The upper terminal contacts of the power semiconductor components are connected to a separate portion of the upper metal layer 2 by means of a bond wire 16, for example. Electrical inlets 13 that are connected to external contacts for tapping the operating voltage are attached to the upper metal layer 2 by means of further soldered joints 12. The arrangement is situated in a housing 7 that is preferably made of plastic and filled with a casting compound 6. The casting compound 6 serves to electrically insulate the arrangement from the environment.
FIG. 3 herein shows the typical course of the equipotential surfaces on the basis of the cross sections that are given in the area of the edge of the substrate due to the equipotential lines 11. The lateral edges of the metal layers 2, 3 do not coincide with the lateral edge 23 of the ceramic layer 1. Given a typical arrangement, the lateral edge of the upper metal layer 2 is more remote from the lateral edge 23 than the lateral edge of the lower metal layer 3. Therefore, the substrate is fashioned similar to a base. The equipotential lines 11 come out of the first main side 21 of the ceramic layer 1, which is provided with the upper metal layer 2 (as shown in FIG. 3). The electrical field intensity is especially high in the edge region of the metal layers 2, 3 as evidenced by the gradient of the equipotential surface lines, and can reach extreme peak values. These high occurring field intensities at the edge regions of the metal layers are the cause for a high partial discharge of the unit, which, as has been experimentally proven, suddenly begins when a threshold value of the operating voltage has been exceeded. The unit, particularly the casting compound 6, is permanently damaged as a result of such a high partial discharge. Therefore, the high-voltage resistance of the unit is limited.
Present further developments of the IGBT units are directed toward high operating voltages, as they are known from thyristors. The voltages between the casting compound and the electrically active part of the unit assume values above 10 kV, so that more stringent requirements must be met with respect to the insulating power of the units and their longevity.